Generally, a piezoelectric substance is made into different piezoelectric elements according to different kinds of objects, and the elements are widely used as functional electronic components, particularly including an actuator producing deformation by the application of a voltage and a sensor generating a voltage based on the deformation of its element in reverse. As a piezoelectric substance used for the applications of actuators and sensors, Pb-based dielectrics having a large piezoelectric characteristic, particularly perovskite type ferroelectrics of the Pb(Zr1-xTix)O3 family referred to as PZT, have been widely used until now, and they are usually formed by sintering oxides composed of individual elements.
At present, as the trend toward smaller size and higher performance of various electronic components advances, smaller size and higher performance also are being strongly demanded of the piezoelectric elements. However, in piezoelectric materials made by a manufacturing method centering on the sintering method, a conventional manufacturing method, as the thickness thereof is made thin, particularly as the thickness approaches a thickness of about 10 μm, it gets nearer to the size of the crystal grains constituting the materials, and thus the effect of the crystal grains becomes unable to be neglected. For this reason, there arises a problem that variations and deterioration in the characteristics become remarkable, and therefore, in order to avoid this problem, a method of forming a piezoelectric substance in which a thin film technology or the like are applied as the replacement of the sintering method has been studied in recent years.
Up to now, as a method of forming a thin film having piezoelectric i.e. a piezoelectric thin film, chemical vapor deposition methods evaporating and vaporizing materials constituting a piezoelectric substance for depositing them on a substrate, generally a rf sputtering method and a MOCVD method, have been studied. However, in piezoelectric thin films formed by these techniques, there are large variations in their characteristics caused by the problems of grain boundary and crystalline orientation, when compared with the piezoelectric characteristics of sintering-formed piezoelectric materials referred to as bulk materials. Further, due to the effects of stress applied to the piezoelectric thin films, etc., the piezoelectric thin films have not yet reached a point where they can provide a piezoelectric characteristic enough to be practically used for sensors and actuators or the like.
Particularly, in order to realize a micro-actuator and a micro-sensor using a piezoelectric thin film, the forming of a piezoelectric element having a bimorph or unimorph structure is considered to be one of its applications. However, in this case, the value of a piezoelectric constant d31 is important as the piezoelectric characteristic. To be put to practical use as a micro-element, the piezoelectric thin film requires the value of the piezoelectric constant d31 equal to or more than −100 pC/V, which is the same value as that of bulk materials. Also, in case of considering mass-production characteristics, a substrate compatible with a process for making the other elements is required to be used for the piezoelectric thin film. Thus, the development of materials for a piezoelectric thin film and the development of a manufacturing technology therefor combined with the realization of high piezoelectric are required.
However, in forming a piezoelectric substance into a thin film, for example, in case of forming piezoelectric substances collectively called PZT into a thin film, treatment at high temperatures or alternatively thin film growth on a substrate at a high temperature equal to or higher than 500° C. needs to be performed, and thus large internal stress is to be left in the formed piezoelectric thin film during the process of cooling after the film formation. The residual stress causes the crystal structure of the film to be greatly different from a bulk material in quality, and often causes degradation in its piezoelectric characteristic and variations in its characteristic. From such reasons, the piezoelectric thin film is difficult to design as an actuator and a sensor, and thus there is not an example that a piezoelectric thin film has been put to practical use.
In case of using the piezoelectric thin film as a micro-actuator or a micro-sensor, it is required to realize a high piezoelectric characteristic and also to decrease variations in the characteristic. Usually, the characteristic of the piezoelectric thin film greatly depends on the compositional structure of thin film materials and the crystal structure thereof. Further, the crystal structure of the piezoelectric thin film greatly depends on a substrate for forming it and a film-forming process. When the piezoelectric thin film is applied as a piezoelectric element, it is required to obtain a high piezoelectric characteristic with stability even under an environment peculiar to the thin film, such as the above internal stress, and thus a process of making thin film and the development of a martial suitable for the thin film are demanded.